In the processing of wood chips preparatory to introduction to a digester, it is preferred to reprocess chips which are thicker than a predetermined thickness and to discard those chip particles which have fibers shorter than a preset minimum length or which are in the form of flakes thinner than a preset thickness, because these are considered to be poor digesting materials. For purposes of the present description, the chips to be reprocessed will be called "over-thick" and the undesired chip particles and flakes will be called "fines."
Chips in excess of 8 mm in thickness tend to remain crude after cooking in the digester, and therefore require after-treatment. Accordingly, it is important to screen out the over-thick chips from the pulp chip supply. The difficulty in accomplishing such screening is compounded by the fact that the chips normally vary in length from about 20 to 30 mm and in width from about 15 to 20 mm. Thus, the thickness of the chips is usually considerably smaller than the other dimensions.
The traditional screening apparatus for pulp chips have been (a) sloped, vibratory holed screens given an oscillating or circular motion commonly in the range of 2 to 3 inches, at a relatively high speed to shift the properly sized chips through the holes in the screen, and (b) disk screens such, for example, as shown in U.S. Pat. No. 4,301,930, which comprises a bed of parallel, corotating shafts carrying interdigitated disks having a clearance defined by the maximum chip thickness to be tolerated.
Disk screens have been considered by many in the cellulose industry as superior to vibratory screens, but as indicated in U.S. Pat. No. 4,660,726, disk screens have a relatively low screening capacity per square meter of screening surface, and, as indicated in U.S. Pat. No. 4,538,734, it is very difficult to attain and maintain uniform slot widths between the disks of a disk screen, particularly when the slot widths are required to be so narrow. As a consequence there have been efforts to provide improved techniques for mounting and replacing the disks of disk screens and attempts to develop a suitable alternative to disk screens. Such attempts have included oscillating bar screens, such as shown in U.S. Pat. No. 4,660,726 and synchronously driven, intermeshing screw spirals, such as disclosed in U.S. Pat. No. 4,430,210.